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Cite this:DOI: 10.1039/c5ee01290b

Electrochemical tuning of olivine-type lithiumtransition-metal phosphates as efficient wateroxidation catalysts†

Yayuan Liu,a Haotian Wang,b Dingchang Lin,a Chong Liu,a Po-Chun Hsu,a Wei Liu,a

Wei Chena and Yi Cui*ac

The oxygen evolution reaction is of paramount importance in

clean energy generation and storage. While the common approach

in search of active, durable and cost-effective oxygen evolution

catalysts involves the development of novel materials, it is equally

important to tune the properties of existing materials so as to

improve their catalytic performance. Here, we demonstrate the

general efficacy of electrochemical lithium tuning in organic electro-

lyte on enhancing the oxygen evolution catalytic activity of olivine-

type lithium transition metal phosphates, a widely-researched family

of cathode materials in lithium ion batteries. By continuously extract-

ing lithium ions out of lithium transition metal phosphates, the

materials exhibited significantly enhanced water oxidation catalytic

activity. Particularly, the electrochemically delithiated Li(Ni,Fe)PO4

nanoparticles anchored on reduced graphene oxide sheets afforded

outstanding performance, generating a current density of 10 mA cm�2

at an overpotential of only 0.27 V for over 24 h without degradation in

0.1 M KOH, outperforming the commercial precious metal Ir catalysts.

The surging global demand for energy, along with the depletionof fossil fuels and the associated adverse environmental impact,has stimulated intense research efforts on sustainable energyconversion and storage systems.1 A great number of promisingenergy conversion technologies, such as water electrolyzers, solarfuel synthetic reactors and rechargeable metal-air batteries, arelimited by the sluggish kinetics of the oxygen evolution reaction(OER).2–7 Even with the state-of-the-art catalysts, namely IrO2

and RuO2, a substantial overpotential (Z) is still needed toachieve practical operating current densities (B0.3 V to reach10 mA cm�2 current).8 In addition, the high cost and the scarcityof such precious-metals further impede their implementation ona large scale. Therefore, the importance of developing highly

efficient and durable OER electrocatalysts that are based exclu-sively on earth-abundant elements can hardly be overemphasized.

Various non-noble metal based OER catalysts have beenreported as competitive alternatives, including first-row transitionmetal oxides and their derivatives,9–16 perovskites,17,18 layereddouble hydroxides,19–23 carbon-based nonmetal catalysts,24–26

as well as amorphous Co–phosphate and Ni–borate materials.27,28

Nevertheless, most of the nonprecious metal catalysts developedso far still underperform the noble metal benchmark. While thecontinuous search for novel catalytic materials with optimizedcomposition and structure is undoubtedly crucial, it is equallyimportant to controllably tune the properties (such as morpho-logies, oxidation states, electronic structures and so on) ofthe existing materials so as to improve their catalyticperformance.20,29–31 Compare to other tuning methods, electro-chemical lithium tuning by means of lithium ion intercalationand extraction in battery cells represents a unique, promisingtuning strategy, due to the fact that the charge–discharge of abattery cell is a dynamic process that spans a wide potential range.Therefore, by precisely controlling the charge–discharge potential,we can tune the materials in a continuous manner in order to

a Department of Materials Science and Engineering, Stanford University, Stanford,

CA 94305, USA. E-mail: yicui@stanford.edub Department of Applied Physics, Stanford University, Stanford, CA 94305, USAc Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator

Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ee01290b

Received 26th April 2015,Accepted 1st May 2015

DOI: 10.1039/c5ee01290b

www.rsc.org/ees

Broader contextWater electrolyzers, solar fuel synthetic reactors and rechargeable metal–air batteries are promising technologies to meet the future demand forclean and sustainable energy resources. However, all of these technologiesare severely limited by the sluggish kinetics of the oxygen evolutionreaction (OER). As a result, the development of highly active, durable wateroxidation catalysts that are based on earth-abundant elements is ofparamount importance. In the past decade, the research in the area ofOER catalysts has been focused on the development of new materials withnovel chemistries as well as structures. However, it is equally important totune the properties of existing materials so as to enhance their catalyticperformance. This paper shows that by electrochemically extractinglithium from the well-studied battery materials, the olivine-type lithiumtransition metal phosphates, their OER performance can be improveddramatically. The demonstrated general efficacy of electrochemical tuningprovides an orthogonal method in search for better electrocatalysts.

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pinpoint the optimized structures and properties of the catalysts.Our previous studies have demonstrated the effectiveness ofelectrochemical tuning on improving the electrocatalytic activitiesof layered materials, such as MoS2 and LiCoO2.29–31 The successmotivates us to wonder whether electrochemical tuning is effec-tive beyond layered structures and can thus be applied to improvethe catalytic activities of other battery materials. Since there isa large pool of materials available as battery electrodes, thedemonstrated general efficacy of electrochemical tuning couldoffer a brand new direction to obtain efficient heterogeneouscatalysts.

Herein, we demonstrate the general efficacy of electro-chemical tuning on olivine-type lithium transition-metal phos-phates LiMPO4 (M = Fe, Mn, Co, Ni) for improved OERperformance. While LiMPO4 compounds have been widelyresearched as cathode materials for lithium-ion batteries,32 tothe best of our knowledge, there has been no report on them asOER catalysts. By continuously extracting lithium ions out ofsubmicron LiCoPO4 particles (denoted as LCoP) in organicelectrolyte, the material exhibited remarkably enhanced OERperformance in alkaline medium. Noticeably, when LiMPO4

particles were reduced to nanoscale and grown directly onreduced graphene oxide (rGO) sheets, such electrochemicaltuning can afford catalysts with unprecedentedly high OERactivity that are among the best OER catalysts reported sofar. Particularly, the delithiated Li(Ni,Fe)PO4/rGO composite(denoted as De-LNiFeP/rGO) showed the best OER performancewith an overpotential as low as B0.274 V to achieve 10 mA cm�2

current density in 0.1 M KOH and an outstanding long-termstability (10 mA cm�2 over 24 h without obvious degradation),exceeding the performance of the benchmark Ir-based catalysts.

The electrochemical tuning process is schematically illus-trated in Fig. 1. In brief, submicrometer-sized LiMPO4 particlesand LiMPO4/rGO composites were synthesized via sol–gel and apreviously reported two-step approach, respectively.33,34 Theproducts were then drop casted onto carbon fiber paper sub-strate (loading 0.5 mg cm�2) and assembled into a pouchbattery cell configuration acting as the cathode, combined withLi metal as the anode and 1 M LiPF6 in 1 : 1 w/w ethylenecarbonate/diethyl carbonate as the electrolyte. The electro-chemical tuning was carried out by charging the materials todifferent voltages following the reaction LiMPO4 - Li1�xMPO4 +xLi+ + xe� (see ESI†).

We first study LCoP particles since numerous Co-based materialshave been proven to be promising OER catalysts.12–14,17,18,27 Themorphology of LCoP was characterized by scanning electron micro-scopy (SEM, Fig. 2a), which showed that the particles are ofsubmicrometer size. The X-ray diffraction (XRD) pattern(Fig. S1, ESI†) confirmed the phase purity of LCoP. The OERperformance of LCoP before and after electrochemical tuningwas investigated in alkaline medium (0.1 M KOH) in a standardthree-electrode system (see details in ESI†). The potentialrequired to reach a current density of 10 mA cm�2 was chosento compare the performance of the catalysts for the valuerepresents the current density from a device with 12% solarto hydrogen efficiency, which is at the upper end of a realisticdevice.2 As can be seen from Fig. 2c, for LCoP before electro-chemical tuning, a large Z (B550 mV) was needed to reach thecurrent density of 10 mA cm�2 and the anodic currentincreased very slowly by applying higher Z, indicating thepoor OER kinetics of the pristine LCoP. However, when LCoPparticles were charged to increasingly higher potentials duringelectrochemical tuning (denoted as De-LCoP followed by char-ging voltage), the linear sweep voltammetry (LSV) curves leftshifted continuously. Fig. 2d shows the relationship betweenthe average potential at 10 mA cm�2 and the electrochemicaltuning potential in which a pronounced enhancement in OERactivity can be observed beyond 4.9 V vs. Li+/Li, correspondingto the lithium extraction potential. The best OER performancewas obtained by charging LCoP to 5.0 V vs. Li+/Li and nosignificant improvement was seen upon further increasingthe charging potential. The saturation in OER performanceover 5.0 V could be due to the reason that the material hasreached its fully charged state as well as the decomposition oforganic electrolyte at elevated potential that blocks the surfaceactive sites. Table 1 summarizes the parameters describing the

Fig. 1 Schematic illustration of the electrochemical tuning process.

Fig. 2 (a) SEM image of the submicron LCoP particles. (b) A typicalcharging curve of LCoP. (c) LSV polarization curves (iR corrected, vs.reversible hydrogen electrode, RHE) of LCoP charged to different potentials,pristine LCoP and Co3(PO4)2 particles in 0.1 M KOH (loading = 0.5 mg cm�2).(d) Plots of the electrochemical tuning potentials of LCoP versus therequired OER potentials to reach 10 mA cm�2 current density.

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OER performance of the pristine and the delithiated LCoP.Compare to LCoP, the delithiated LCoP above 5.0 V had analmost 130 mV lower Z at 10 mA cm�2, and the Tafel slope wasdecreased from B100 mV dec�1 (pristine LCoP) to B 52 mV dec�1

(De-LCoP 5.0 V). Such a significant improvement confirmed theefficacy of electrochemical tuning on olivine-type LiMPO4 materials.Notably, delithiated LCoP was also much more active for OER thancrystalline Co3(PO4)2 particles of similar size (Fig. S2, ESI†), indicat-ing that the special olivine structure of the De-LCoP could beaccounted for the good OER performance.

It is well-recognized in the battery community that theinherently low ionic and electronic conductivities of LiMPO4

due to the robust covalent bonding of PO43� severely limits the

charge–discharge kinetics of these materials.35 Such disadvant-ages are also believed to be adversely affecting the performanceof LiMPO4 when used as electrocatalysts. In recent years, severalstrategies have been devised to improve the conductivity andstability of LiMPO4 in its charged state, including reducing the

particle size to nanoscale, combining the particles with conduc-tive materials such as carbon as well as doping LiMPO4 withFe.36–38 Following the abovementioned rationale, we synthesizedLCoP and Fe-doped LiMPO4 (for higher electronic conductivity)nanoparticles directly anchored on rGO (denoted as LMP/rGO) inseek of even better OER catalysts.

SEM (Fig. 3d inset, Fig. S3, ESI†), XRD (Fig. S4, ESI†) andenergy-dispersive X-ray spectroscopy (EDX, Fig. S5 and Table S1,ESI†) were conducted to identify the morphology, phase andcomposition of the synthesized LMP/rGO hybrid materials respec-tively. The content of rGO was measured to be approximately15 wt% via thermogravimetric analysis (TGA, Fig. S6, ESI†). All thesamples were charged to various potentials vs. Li+/Li duringelectrochemical tuning (Fig. S7–S11, ESI†). Consistent with LCoP,the OER polarization curves shifted to the left for the all sampleswith increasing tuning potential. Appreciable improvements inOER performance can be observed above 4.80 V (Co2+/3+) for LCoP/rGO and LCoFeP/rGO, above 4.30 V (Mn2+/3+) for LMnFeP/rGO and

Table 1 Summary of the OER activities of LCoP charged to various potentials

SampleAverage potential at10 mA cm�2 (V vs. RHE) Tafel slopes (mV dec�1) Sample

Average potential at10 mA cm�2 (V vs. RHE) Tafel slopes (mV dec�1)

De-LCoP 5.1 V 1.654 77 � 1 De-LCoP 4.9 V 1.691 73 � 5De-LCoP 5.0 V 1.655 52 � 5 De-LCoP 4.8 V 1.698 83 � 6De-LCoP 4.98 V 1.662 67 � 3 Pristine LCoP 1.781 100 � 2De-LCoP 4.95 V 1.668 69 � 2 Co3(PO4)2 1.791 109 � 3

Fig. 3 (a, b) LSV polarization curves of the pristine and delithiated LMP/rGO hybrids in 0.1 M KOH (loading = 0.5 mg cm�2). The electrochemical tuningpotentials are 5.0 V for De-LCoP/rGO and De-LCoFeP/rGO, 4.5 V for De-LMnFeP/rGO and 5.1 V for De-LNiFeP/rGO and De-LCoNiFeP/rGO. (c) OERpotentials at 10 mA cm�2 current density of the various De-LMP/rGO in 0.1 M KOH (loading = 0.5 mg cm�2). (d) LSV polarization curves of De-LNiFeP/rGO and Ir/C in 0.1 and 1 M KOH. The inset is the SEM image of LNiFeP/rGO. (e) Tafel plots of De-LNiFeP/rGO and Ir/C in 0.1 M KOH.(f) Chronopotentiometry curves of De-LNiFeP/rGO and Ir/C at the current density of 10 mA cm�2 in 0.1 and 1 M KOH.

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above 5.00 V (Ni2+/3+) for LNiFeP/rGO, corresponding to thepotential of the M3+/2+ redox couples of the major transition metalelements in these materials.39 Noticeably, there are two distinctimprovement steps for LCoNiFeP/rGO (Fig. S11, ESI†), which areabove 4.80 V and 5.00 V, corresponding to the redox potential ofCo2+/3+ and Ni2+/3+ respectively. Fig. 3a and b show the best OERpolarization curves of the delithiated LMP/rGO hybrids (denotedas De-LMP/rGO) in comparison with their pristine counterparts.For all the samples, the potential at 10 mA cm�2 was decreaseby around 65–130 mV after the electrochemical tuning process(Fig. S7–S11, ESI†), demonstrating the effectiveness of electro-chemical tuning on improving the OER performance of olivine-type LiMPO4. Finally, the performance of the De-LMP/rGO hybridswas better than the submicron-sized LCoP, which confirmed thatthe reduced size, the introduction of Fe and the association withrGO did facilitate the charge transport of LiMPO4.

It can be seen from Fig. 3c that De-LNiFeP/rGO yielded thebest OER performance with a potential as low as 1.50 V vs. RHEat 10 mA cm�2, which was among the most active non-preciousmetal OER electrocatalysts (Table S3, ESI†). For comparison, acommercial Ir/C catalyst (20 wt% Ir on Vulcan carbon blackpurchased from Premetek Co.) with the same loading was alsotested (Fig. 3d). In 0.1 M KOH, De-LNiFeP/rGO exhibited alower onset potential (B1.45 V vs. RHE) than Ir/C (B1.50 V vs.RHE). In addition, compare to Ir/C, the Z of De-LNiFeP/rGO at10 mA cm�2 was reduced by 45 mV and the anodic currentdensity increased more rapidly. In 1 M KOH, the onset potentialof De-LNiFeP/rGO was reduced to B1.47 V vs. RHE, greatlyoutperforming that of Ir/C (B1.50 V vs. RHE). Note that thepeak around 1.43 V of De-LNiFeP/rGO in 1 M KOH is attributedto the Ni oxidation process.40 Moreover, the catalytic kineticsof the two catalysts were examined by Tafel plots (Fig. 3e).

The Tafel slope of De-LNiFeP/rGO is approximately 57.5 mV dec�1

in 0.1 M KOH, which is lower than that of Ir/C (B65.4 mV dec�1),suggesting its favorable reaction kinetics. In addition to highOER catalytic activity, De-LNiFeP/rGO also exhibited excellentstability in alkaline solutions (Fig. 3f). When galvanostaticallybiased at 10 mA cm�2 in 0.1 M KOH, the catalyst has a nearlyconstant operating potential for over 13 h (B7 mV increaseafter 13 h) while the operating potential of Ir/C increasedconsiderably (420 mV) after 13 h. In 1 M KOH, De-LNiFeP/rGO operated continuously for 24 h with only B14 mV increasein operating potential, whereas the Ir/C catalyst decayed rapidlywithin only 6 h (B50 mV increase). These results indicate thatLNiFeP/rGO hybrid after electrochemical tuning can afford OERcatalysts of high efficiency and durability outperforming thecommercial benchmark catalyst.

Various characterizations were carried out to account for theimproved OER performance of LiMPO4 after electrochemicaltuning. First of all, the morphology of the catalysts afterdelithiation was characterized by SEM (Fig. S12 and S13, ESI†),from which no obvious changes can be observed. Therefore, theenhanced catalytic activity was unlikely the result of morpho-logical changes induced by the electrochemical tuning. How-ever, the electrochemically active surface area (ECSA) of thecatalysts after tuning increased considerably as estimated frommeasurements of the electrochemical double-layer capacitance(EDLC). The EDLC of LCoP was only B1.25 mF cm�2 whereasthat of De-LCoP was B11.20 mF cm�2 (Fig. 4a–d), whichincreased by nearly 10 times after delithiation. Similarly, allthe LMP/rGO hybrids after electrochemical tuning exhibitedincreased EDLC (Fig. S14 and Table S2, ESI†) of B1.5–4 times.The result indicates that the number of electrochemically activesites for water oxidation was increased significantly due to

Fig. 4 (a, c) EDLC curves of LCoP and De-LCoP 5.0 V respectively with different scan rates; (b, d) plots of current densities at 0.25 V versus scan rates ofLCoP and De-LCoP 5.0 V respectively. (e) Powder XRD patterns of LCoP, LCoP 4.8 V and LCoP 5.0 V. (f) Localized XRD patterns of LCoP and LCoP 5.0 Vwith 2y ranging from 28–381. (g) Li/Co atomic ratio of LCoP, LCoP 4.8 V and LCoP 5.0 V obtained by ICP-MS.

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electrochemical tuning, shedding light on the improved OERperformance. Noticeably, there exists a positive correlationbetween the electrochemical tuning potential and the EDLC(Fig. S17, ESI†). With increasing electrochemical tuningpotential, the EDLC area of LCoP increases accordingly. Thesaturation in EDLC above 5.0 V, as mentioned previously, couldbe attributed to the fact that the material is fully charged as wellas the decomposition of the organic electrolyte at high voltagethat blocks the active surface sites. Therefore, continuouslyextracting Li ions will create more active sites for OER. As theincrease in EDLC for LCoP is the most significant, it showsthe greatest amount of overpotential reduction at 10 mA cm�2

(126 mV). In addition, as can be observed from the XRD spectra(Fig. 4e), the crystal structure of the olivine-type LiMPO4

catalysts remained unchanged after electrochemical tuning,which is consistent with existing studies.41–43 Nevertheless, aslight right-shift of the XRD peaks can be observed due to theshrinking lattices after delithiation (Fig. 4f),44,45 and the induc-tively coupled plasma-mass spectroscopy (ICP-MS) analysisfurther corroborated the extraction of Li+ by electrochemicaltuning (Fig. 4g and Fig. S15, ESI†). Thus, unlike the crystalstructures of the previously reported phosphate based OERcatalysts and the Co3(PO4)2 control in this work,46,47 such anunique lithium-deficient transition metal phospho-olivinestructure that can be obtained only via electrochemical routemay be responsible for a higher catalytic activity.48 Moreover, itis believed that the transition metals M (2+) in LiMPO4 wereoxidized for charge compensation when lithium ions wereextracted upon electrochemical tuning.49,50 To confirm theincrease in valency, X-ray photoelectron spectroscopy (XPS)was performed on the catalysts before and after delithiation(Fig. S20, ESI†). As can be observed from the spectra, the XPSpeaks of the transition metals all shifted to higher bindingenergy after delithiation, indicating the increased oxidationstates of the metal centers. Since transition metal centers ofhigh valency that are capable of buffering the multi-electronprocesses necessary for water oxidation are generally regardedas catalytically active sites,51–54 the enhanced OER performanceof the delithiated LiMPO4 is also originated from the increasedoxidation state of the metal centers. Finally, in an importantstudy by the Shao-Horn group, a design principle for OERcatalysts was established that materials with the most activeOER activity shall be the ones with high covalency of transitionmetal–oxygen bonds.17 Larger O-2p character of the active Mcenter was reported to promote the surface charge transfer toabsorbates (e.g. O2

2� and O2�), which is the rate determiningstep in OER. Such principle has been successfully applied tovarious transition metal OER catalysts systems,55,56 and webelieve is also insightful in explaining the enhanced OERperformance of LMP after electrochemical tuning, from atheoretical perspective. From previous studies, delithiation ischaracterized by the shift of the M-3d bands to lower energiesand the increased covalent hybridization between the M-3d andO-2p states.57 The phenomenon is especially pronounced fordelithiated Ni containing LMP,58 further explaining the notableOER performance of De-LNiFeP/rGO.

In summary, we employed an electrochemical tuning(delithiation) strategy in organic electrolyte to effectively improvethe OER performance of olivine-type LiMPO4 in aqueous solution.Noticeably, the LNiFeP/rGO hybrids upon tuning exhibited unpre-cedentedly high OER electrocatalytic performance in alkalinesolutions, outperforming the commercial Ir/C catalyst in bothactivity and stability. The improved OER performance could beattributed to the synergistic effect of the increased electrochemi-cally active site density, the higher valency of the transition metalcenters and the increased covalent hybridization between theM-3d and O-2p states. The study represents a successful appli-cation of electrochemical tuning beyond layered materials andthe clearly demonstrated general efficacy of the methodologyopens up new venue to fine tune the existing materials for moreefficient and durable heterogeneous catalysts in the area ofenergy research.

Acknowledgements

This work was initiated by the support of the Department ofEnergy, Office of Basic Energy Sciences, Materials Sciences andEngineering Division, under contract DE-AC02-76-SFO0515.This work is supported by Global Climate and Energy Projectat Stanford University. We acknowledge Professor MatthewW. Kanan and Mr Xiaoquan Min for the help on the gaschromatography measurements.

Notes and references

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